1. Field of the Invention
This invention relates to a power supply device for efficiently supplying power to a load by reducing the internal power consumption of the device, and to an electronic appliance equipped with such a power supply device.
2. Description of the Related Art
An electronic appliance such as a notebook type personal computer contains a secondary battery such as a NiCd battery as a power supply device. The appliance can also utilize a D.C. voltage, obtained by rectifying a commercial A.C. power supply, as an external D.C. power supply. A charger is provided to the appliance so as to charge the secondary battery from the external D.C. power supply. When the external D.C. power supply is utilized, an A.C. adaptor for rectifying the commercial A.C. power supply and obtaining the D.C. voltage is connected to the power supply device. The secondary battery and the external D.C. power supply are connected in parallel across the load. Back-flow prevention diodes are interposed between the secondary battery and the load and between the external D.C. power supply and the load lest a current flows from the secondary battery to the external D.C. power supply or from the external D.C. power supply to the secondary battery. When the external D.C. power supply is inputted, the voltage of the external D.C. power supply is set to a level higher than that of the voltage of the secondary battery. Therefore, the supply of power to the load is made from the external D.C. power supply. When the external D.C. power supply is inputted and the voltage of the secondary battery drops below a predetermined value, the charger charges the secondary battery. When the A.C. adaptor is removed and the input of the external D.C. power supply is cut off, the supply of power to the load is made from the secondary battery.
In the power supply device according to the prior art described above, the back-flow prevention diode is interposed between the load and the secondary battery. Therefore, a load current flows through this back-flow prevention diode when power is supplied from the battery to the load, and a power loss develops in the back-flow prevention diode. This power loss in the back-flow prevention diode cannot be neglected in consideration of the battery capacity. The voltage drop of the diode is as great as from 0.55 to about 0.7 V. When six NiCd batteries are used as the secondary battery, the power loss reaches 7.6 to 10% of the battery capacity. When the battery consists of two NiCd batteries, the proportion of the power loss becomes further higher, and reaches 23 to 30% of the battery capacity.